1. Field of the Invention
The invention relates to a focus control apparatus and a focus control method and, more particularly, to focus control apparatus and method for performing a control operation to attain an optimum converging position of read light which is irradiated to a predetermined recording surface of a recording medium, with respect to the recording surface.
2. Description of Related Art
Nowadays, an information recording medium of high recording density and a large capacity, called a digital video disc or digital versatile disc (abbreviated as a DVD) and a system using this medium are being used widely. As such a disc, there is a type in which information recording surfaces are formed in upper and lower two layers which sandwich an intermediate layer (spacer or space region). To read the disc of this type by an optical pickup from one of the disc surfaces, a focal point (focal position or optimum converging position) of the read light has to be positioned on the information recording surface in a desired one of the layers.
Hitherto, in a reading system of a well-known compact disc (CD) having an information recording surface in a single layer, an initial focus control procedure described below is conventionally adopted in a set-up mode to again read the disc just after a power source of a player is turned on, just after a disc has been loaded into a disc player, or the like. In such an initial focus control procedure, the focal point of the read light is moved from a position away from the recording surface of the disc to be read so as to approach the recording surface, a focus servo loop is closed at a predetermined zero-cross point in an S-shaped waveform of a focus error signal which is generated based on an output of a pickup during the movement, then a tracing operation of the read light focal point to the recording surface is started.
FIG. 1 shows a relationship between the position, in an inner layer of the disc, of a focal point P of the read light which is determined by an objective lens 100 and a level FE of the focus error signal which is obtained when the focal point P is moved in the optical axial direction. It will be understood that the focus error signal fundamentally draws an S-shaped curve around a zero level (zero-cross point ZC00, ZC10) that is obtained in a state where the focal point P is positioned on the recording surface, wherein the zero level corresponds to a center of the S-shaped curve. A period of time between the minimum value and the maximum value of one S-shaped curve almost corresponds to a control range of the focus servo loop.
The initial focus control in this case is performed in a manner such that when the first recording surface on the objective lens side is read, the focus servo loop is closed in response to a detection timing of the zero-cross point ZC00 and, when the second recording surface on the inside is read, the focus servo loop is closed in response to a detection timing of the zero-cross point ZC1O.
With respect to the feature that the focus servo loop is closed when the focal point of the read light is positioned on the recording surface to be read as mentioned above, the reading of the double-layer recording disc is the same as that of the conventional CD. In the single-layer type recording disc like a CD and the multilayer type recording disc, however in particular, since the recording surface's light reflectance differs largely between the single-layer type recording disc and the multilayer type recording disc, a situation occurs such that a gain of the focus servo loop for the recording surface of the disc of one of the types and a gain for the recording surface of the disc of the other type are remarkably different. In this situation, even if the focusing servo adapted to the disc of one type is applied to the disc of the other type, it takes relatively a long time for settlement of the servo operation after the focus servo loop is closed. In the worst case, the read light cannot be focused on the target recording surface. In this way, failure of the focusing set-up operation can be resulted.
The above problem shall also apply to each of one and the other recording surfaces of the multilayer type recording disc. Furthermore, it occurs not only by the difference of the light reflectance between the recording surfaces but also by fouling of the objective lens in association with the aging change or the like or deterioration of the other optical systems.